Continue working on handling of properties in bidi iteration.

Continue working on handling of properties in bidi iteration.
Region display and extension seems to work.
Solved a crash in bidirectional display of etc/HELLO.
(HELLO display still not 100% OK, e.g. near Kannada.)

 .gdbinit (pitx): Display some bidi information about the
 iterator.

 dispextern.h (BIDI_AT_BASE_LEVEL): Enclose definition in
 parentheses.

 xdisp.c (handle_stop_backwards): Save and restore it->current
 and it->position, instead of expecting the caller to do that.
 (next_element_from_buffer): When moving across stop_charpos,
 record it in prev_stop.  When IT_CHARPOS backs up, call
 handle_stop_backwards only if above the base embedding level.
 This solves the crash while displaying etc/HELLO in bidi mode.